Commercial portable radio devices, such as mobile phones, include models with a total device depth of about one centimeter, for example. Such flat structures are especially the folding parts of flip-type mobile phones. A flip phone has got two parts such that the parts can be folded over, on a hinge, so that they lie on top of each other or adjacently end-to-end in almost the same plane. In the first position, the device is particularly small, and it is in the latter position during connection.
Antennas used in flip phones are normally monopole-type external antennas. Their drawback is the inconvenience generally associated with a protruding structural element. Naturally it would be possible to use internal PIFA-type planar antennas, but the thin structure of the folding parts in the mobile phone would result in the distance between the radiating part and ground plane to be so small that the antenna gain would be unsatisfactory. Furthermore, it would be possible to have an internal monopole-type planar antenna such that the radiating plane does not face the ground plane. In that case the flatness of the device would cause no problem as such, but the electrical characteristics such as matching and antenna gain would again be unsatisfactory. Matching could be improved using an additional circuit, but this would require the use of several discrete components.